This invention relates generally to blast furnaces and to their operation. More specifically the invention relates to the structure of the hearth bottom of a blast furnace and to a method for operating a blast furnace that protects the hearth bottom and provides enhanced flexibility to the operation of a blast furnace.
The hearth of a conventional blast furnace is usually made of refractory material, which, in the course of use, grows increasingly thinner as a result of chemical attack from the molten iron and slag thereon and as a result of thermal wear from the intense blast furnace heat.
There is normally provided on the bottom of the hearth a steel plate (hereinafter referred to as the "bottom plate") to keep the furnace well sealed. As the refractory material thins the thermal load on the bottom plate increases and it may become thermally deformed or worn off, thereby rendering normal blast furnace operation impossible.
Even if such thermal deformation or wearing of the bottom plate is avoided, a concrete foundation supporting the furnace structure may become heated and weakened, resulting in the deformation, or even breakdown, of the furnace structure. Such deformation or breakdown of a concrete foundation would also render the maintaining or continuing of normal furnace operation impossible.
Various arrangements for preserving the hearth bottom have been proposed. Such arrangements have included cooling the hearth bottom by providing a set of cooling fluid passages (hereinafter referred to as a cooling pipe) between the hearth bottom and the concrete foundation and regulating the quantity and/or type of cooling fluid supplied therethrough, or supplying different coolants, according to the thermal load working on the hearth bottom. Such an arrangement is disclosed in Japanese Patent Publications No. 10683 (1965) and No. 810801 (1976), and Japanese Patent Application Publication No. 74908 (1976).
The cooling arrangement such as those described in the above-mentioned references is not effective enough to provide adequate cooling to the hearth bottom because the amount of cooling cannot be adequately controlled. The flow rate of the cooling fluid (such as a mixture of water and air) cannot be increased freely with increasing thermal load on the hearth bottom because of the limit of the cooling pipe diameter or because of the capacity of the coolant supply unit providing the coolant.
Conventional cooling arrangements may include packing material, having good thermal conductivity packed around the cooling pipes to promote cooling. When the thermal load on the hearth bottom decreases, the cooling effect is lowered by changing the cooling fluid or reducing the fluid supply accordingly. However, when the hearth is not cooled, heat from inside the blast furnace is conducted through the packing material to the concrete foundation. This heats and weakens the concrete foundation, possibly leading to deformation or breakdown of the blast furnace structure.
As the size of the blast furnace change with a changing steel industry, greater flexibility in hearth bottom cooling arrangements are required. A conventional steelworks used to operate five to six mediumsized blast furnaces, each having a working volume of approximately 2000 m.sup.3. With a more economical and efficient mass production in view, it has recently become a common practice to operate two or three 4000 m.sup.3 or larger blast furnaces capable of producing more than 10,000 tons of pig iron per day. With the steel industry getting used to this new practice, the larger blast furnaces have proved effective in establishing stable low-cost iron production, lowering the fuel ratio from 500 kg to 400 kg per ton of pig iron produced.
However, unavoidable shutdowns of such a large blast furnace necessitates production increases in the remaining blast furnaces. On the other hand, when the industry faces a contraction of demand for steel, production must be curtailed sharply over a long period of time. Under such circumstances, a steelworks operating two or three extra-large blast furnaces has to make as great a production increase or decrease as is comparable to the production capacity of a conventional medium-sized blast furnace.
Generally, however, a blast furnace is designed to have a hearth bottom cooling capacity that is based on the thermal load working on the hearth refractory when the furnace is producing pig iron at full capacity. In addition to being designed for maximum load, the flexibility of the cooling capacity, particularly in the lower range, usually is very limited.
When fuel consumption is reduced to meet a sharp production cut as mentioned before, therefore, the hearth bottom is overcooled so that there arises an abnormal solidification of the molten product at the upper surface of the hearth bottom and a resulting bulging thereof. This leads to unstable production reduction and inefficient furnace operation. Thus, there is a need for a cooling arrangement providing sufficient flexibility to deal with a wide range of production level.